Magnetic deflecting system for cathode-ray tubes



April 1950 C-,N. SMYTH 2,505,011

mcms'rxc max-"menus SYSTEM FOR CATHODE-RAY TUBES Filed Aug. 19', 1947 Inv nlor 4 4% W A ltorney Patented Apr. 25, 1950 MAGNETIC nnrtecrme SYSTEM roe...

GATHQDE-RAYTUBES Charles Norman Smyth, London, England, assignor to International Standard Electric Corporation, New York Delaware N. Y., a corporation of Application August 19, 1947, Serial No. 769,552 In Great Britain August 9, 1939 I station 1, Public Law 690, August s, 1946 Patent expires August 9, 1959 4 Claims. (01. 315-27) This invention relates to magnetic deflecting systems for cathode ray tubes for use for example in television receivers.

In the arrangement of the invention two crossed magnetic fields are caused to vary simultaneously to give a resultant field for deflection in one direction and are also caused to vary simultaneously but in a different relative sense to give a superposed resultant field for deflection in the transverse direction. The invention in one aspect resides in a coil structure with or without a magnetic core in which the coils are arranged and inter-connected to permit the required fields to be set up when suitably excited. In another aspect the invention resides in a supply circuit adapted to feed the necessary currents to a coil structure to set up the required magnetic fields.

The invention will be more fully explained as embodied in one practical arrangement and with reference to the drawings left with the provisional specification.

It is common practice in television receivers to employ a system of four coils around the neck of the cathode ray tube to effect the scanning. One pair of coils is used for the line scanning and another for the frame scanning. In Fig. l of the drawings is shown a cross section of the cathode ray tube and deflector coil windings taken through the centre of the deflector coils. The coils A and B could be used for frame scanning and the coils C and D, for line scanning. These windings comprise four separate elements each shown in Fig. 4. tributed around the neck of the tube in the manner shown, in order to provide a uniform magnetic field throughout the useful cross sectional area of the neck.

It is now proposed to employ a simplified form of deflector coil as shown in Fig. 2 which comprises two coils LI and L2 (each as shown in Fig. 4) arranged around the neck of the tube as shown in Fig. 3. The active length of each coil is in practice substantially the same although the spacing between the end loops has been exaggerated in Fig. 3 for the sake of clarity. Alternatively, two coils each as shown in Fig. 5 could be employed if absolute symmetry of the end turns was in special instances found necessary. In this case the end loops are distributed on both sides of the neck. The fields in the several coils due to the line currents should be equal and those due to the frame currents should also be equal. The coils should therefore be as like as The windings are disto practical considerations can be neutralised in effect by increasing or reducing the number of turns in one of the coils. Now if the two coils, section as shown in Fig. 2, were fed separately with scanning currents of line and frame frequency, the resultant raster wouldbe very distorted due to the curvature of the deflecting field near the edges of the cathode ray tube between the two coils. It is proposed, therefore, to overcome this difiiculty and produce a field identical with that produced by the coils of Fig. 1 and still to maintain the simplification of the coils described by using both coils in Fig. 2 for both line and frame deflection, feeding them with currents in the same direction for deflection in one direction and in opposite directions for deflection in the other. This can be arranged circuitally by feeding the coils in series for one deflection and in parallel for the other.

A suitable circuit for feeding the coils is shown in Fig. 6. The deflector coils LI and L2 are connected in series across a line scanning transformer Ti which has a split secondary. The coils are fed in parallel for frame deflection by the transformer T2 connected between the centre tap of the coils and the centre of the line transformer secondary. A tapped resistance (or reactance) R is included to obtain exact balance of the two halves of the circuit. It is noteworthy that from the point of view of line scanning voltages the two ends of the secondary of the frame scanning transformer T2 are at the same potential and so no line scanning currents influence the frame scanning circuit. Additionally the frame scanning currents pass through the two halves of the line scanning transformer secondary equally and oppositely and therefore produce no resultant M. M. F. in this quarter: consequently it is only the resistance and not the inductance of the line transformer secondary which is a series inefficiency in the frame scanning circuit, and the core of the line scanning transformer is not disturbed from its normal magnetic working point by the fluctuating frame scanning currents.

Modifications of the coil structures and circuit illustrated will be readily appreciated. Obviously the line scanning currents may be fed in parallel and the frame scanning currents in series. The coils may be connected in series and symmetrically in the output circuit of a pushpull amplifier, to which the scanning waves for one direction are fed in push-pull and the scanning waves for the other direction in parallel.

possible but any differences in dimensions due Thus a common amplifier can be used for both line and frame currents. Instead of bunching the turns as shown in Fig. 4, the turns may be distributed over the neck to modify the distribution of the lines of force within the tube. It will also be seen that coil structures of the kind at present in use, such as that referred to with reference to Fig. 1 can be interconnected and excited to function in accordance with the invention.

What is claimed is:

1. An electromagnetic system for deflecting an electron beam in a cathode ray tube in two directions at substantially right angles, comprising a coil structure in which two coils are arranged around the neck of said tube, means to feed said coils in series to deflect said beam in one direction, a second means to feed said coils in parallel to deflect said beam in the other direction, said two means being interconnected.

2. A deflector coil structure for the system of claim 1 comprising two crossed series-connected coils adapted to embrace the neck of the cathode ray tube with their axes perpendicular to the tube axis.

CHARLES NORMAN SMYTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,007,380 Morlock July 9, 1935 

